This invention relates to a process for continuously conveying granular solids from a first zone at a pressure of 4 to 16 bar through a descending line and via an ascending line to a second zone with a pressure which is lower than that in the first zone by 3 to 15 bar, by means of a gaseous medium.
In a direct-reduction plant, the granular solids, e.g. direct-reduced iron, are brought from an elevated pressure to ambient pressure for further processing in a plant. In this plant, hot direct-reduced iron is conveyed from a fluidized-bed reactor to a pressurized cyclone, from which the granular solids are discharged downwards via a storage bin through a descending line. Inert gas is introduced into the descending line.
The descending line is usually connected by a valve with an ascending line through which the granular solids are conveyed upwards into an impact pot by supplying inert gas into the ascending line. From the impact pot, the solids are introduced into a briquetting bin and inert gas is also introduced. Due to the continuous supply of fine-grained solids, a column of these solids is formed in the descending line and forms a pressure barrier.
When the valve is used, the point of transition from the descending line to the ascending line requires much effort and maintenance, as due to the temperature level and the coarse surface of the granular solids the seals and the closure body are subjected to a high degree of wear and therefore must be replaced quite often.
In U.S. Pat. No. 2,684,873, fine-grained solids are introduced into an inflow tank, the amount supplied again being regulated by means of a valve. In the inflow tank, a supply of fine-grained solids is formed, into which extends a tube through which the solids are transported into a tank disposed at a higher level. A line pressurizing the inflow tank opens into the inflow tank, whereby the solids are transported through the tube into the tank disposed at a higher level.
This known process likewise uses valves for regulating the inflow of solids. For larger amounts of solids to be transported, the inflow tank must be dimensioned correspondingly large, which makes this apparatus also very complex and expensive.
It is the object of the invention to provide an improved process for conveying granular solids whereby the pressure between two regions can be reduced inexpensively and with little maintenance of the parts of the apparatus.
In accordance with the invention, this object is achieved in that the inflow of the gaseous medium is effected through an upwardly directed nozzle at the point at which the descending line opens into the ascending line.
In order to lose as little kinetic energy as possible, the nozzle orifice for the inflow of gas should expediently be disposed as close as possible to the bulk material to be transported. Due to the line of fall of the bulk material, the nozzle orifice should therefore advantageously be disposed at a distance of 0.5 to 8 times the hydraulic diameter of the nozzle orifice below the point of intersection of the tube axes.
Since the fine-grained solids flowing out of the descending line are compacted due to the column of solids above the outlet of the descending line it is advantageous to dispose auxiliary nozzles for loosening up the solids around and along the pressure line.
It is particularly advantageous when the solids weight in the ascending line lies in the range from 0.3 to 0.8 times the bulk weight, preferably in the range of 0.4 to 0.7 times the bulk weight.
With a limited overall height it is preferable to convey the solids via a multi-stage decrease in pressure.